1. Field of the Invention
The present invention relates to pulverized coal boilers and, more particularly, to a mechanism for directing coal flow to the corresponding outlet pipes of the vertical spindle mill with negligible effect on the pre-existing primary air flow distribution, the mechanism comprising an array of individually adjustable flow control elements positioned inside the discharge turret of the vertical spindle mill.
2. Description of the Background
Coal fired boilers utilize pulverizers to grind coal to a desired fineness so that it may be used as fuel for burners. In a typical large pulverized coal boiler, coal particulate and primary air flow from the pulverizers to the burners through a network of fuel lines that are referred to as coal pipes. Typically, raw coal is fed through a central coal inlet at the top of the pulverizer and falls by gravity to the grinding area at the base of the mill. Once ground (different types of pulverizers use different grinding methods), the pulverized coal is transported upwards, using air as the transport medium. The pulverized coal passes through classifier vanes within the pulverizer. These classifier vanes may vary in structure, but are intended to establish a swirling flow within the rejects cone to prevent coarse coal particles from flowing into the discharge turret of the pulverizer. The centrifugal force field set up in the rejects cone forces the coarse coal particles to drop back down onto the grinding surface until the desired fineness is met. Once the coal is ground finely enough, it is discharged and distributed among multiple pulverized coal outlet pipes and into respective fuel conduits where it is carried to the burners. Each coal pulverizer is an independent system and delivers fuel (pulverized coal) to a group of burners.
In a conventional coal pulverizer 100 as shown in FIG. 1 (A & B), raw coal 101 is dropped into coal inlet port 102 and by force of gravity falls through coal chute 103 until it reaches the grinding mechanism 104. The grinding mechanism 104 grinds the coal into fine pieces. Air 105 flows into air inlet port 106 through a nozzle ring on the outside perimeter of the grinding mechanism 104, feeding primary air into the pulverizer. This creates a stream of low-velocity air that carries the particles of pulverized coal upward where they enter classifier vanes 109 that establish a swirling flow within a reject cone 120. The centrifugal force field set up in the reject cone 120 prevents coarse pieces of coal 110 from entering the discharge turret 108. The coarse pieces of coal 110 fall by force of gravity back into the grinding mechanism 104. Once the pulverized coal 107 enters the discharge turret 108 it is distributed between the multiple equal diameter pulverized coal outlet pipes 111 (FIG. 1 shows six pulverized coal outlet pipes 111 at the top). The pulverized coal 107 is then carried by connected fuel conduits to a boiler where it is burned as fuel.
FIG. 2 is a simplified cross-section of the vertical spindle pulverizer as in FIGS. 1A & 1B with four outlet pipes, and FIG. 3 is a top view of FIG. 2. Poor balance of pulverized coal 107 distribution between pulverized coal outlet pipes 111 is commonly experienced in utility boilers. This can be due to various reasons, such as system resistance of each individual fuel conduit, physical differences inside the pulverizer, and coal fineness. The unbalanced distribution of coal among the pulverized coal outlet pipes adversely affects the unit performance and leads to decreased combustion efficiency, increased unburned carbon in fly ash, increased potential for fuel line plugging and burner damage, increased potential for furnace slagging, and non-uniform heat release within the combustion chamber. In addition, it is critical for low NOx (Nitric Oxides) firing systems to precisely control air-to-fuel ratios in the burner zones to achieve minimum production of NOx. The relative distribution of coal between the pulverized coal outlet pipes is monitored by either measuring the pulverized coal flow at the individual pulverized coal outlet pipes or measuring the particular flame characteristics of burning fuel discharged from the each of the burners.
The distribution of primary air throughout the coal piping network is controlled by the flow resistances of the various coal pipes. Because of differences in pipe lengths and numbers and types of elbows in each fuel line, the different coal pipes from a pulverizer will usually have different flow resistances. It is known that fixed or adjustable vanes may be used to directly divert the coal flow distribution among the outlet pipes 111. The following references describe the use of vanes to modify coal flow distribution.
U.S. Pat. No. 4,570,549 to N. Trozzi shows a Splitter for Use with a Coal-Fired Furnace Utilizing a Low Load Burner.
U.S. Pat. No. 4,478,157 to R. Musto shows a Mill Recirculation System.
U.S. Pat. No. 4,412,496 to N. Trozzi shows a Combustion System and Method for a Coal-Fired Furnace Utilizing a Low Load Coal Burner.
Finally, U.S. Pat. No. 2,975,001 issued on Mar. 14, 1961 to Davis discloses an apparatus for dividing a main stream of pulverized coal between two branch streams. (Col. 1, lines 50-52). The apparatus may be used alone or in conjunction with a conventional slotted riffle. (Col. 1, lines 70-73). The apparatus is comprised of a combination fixed and tiltable nozzle. (Col. 1, lines 50-58). The fixed nozzle is attached to the main duct leaving the pulverizer and concentrates the coal and air flow (see claims 1-5). The concentrated coal and air flow is then directed into the tiltable nozzle with the highest concentration of coal necessarily being at the nozzle centerline. The tiltable nozzle is then “tilted” in order to direct the concentrated coal and air flow into one or the other branch stream. Guide vanes may be mounted inside the tiltable nozzle; however, this patent does not disclose adjustable guide vanes. (Col. 1, lines 58-60).
All of the foregoing references teach a form of direct diversion of the coal flow, but this likewise causes direct diversion of the air flow. It is impossible using direct diversion to increase or decrease the flow of coal into a particular outlet pipe without effecting primary air flow, or vice versa.
In contrast to an adjustable baffle approach which makes it difficult to simultaneously balance coal and primary air flow rates, the present invention makes it possible to increase or decrease the coal flow in any one of the above-described outlet pipes 111 without affecting the pre-existing air flow distribution among the outlet pipes by changing the position and/or orientation of the control vane in the region of high particle concentration. This unique approach makes it possible to balance the coal flow distribution among the outlet pipes, while eliminating the need to readjust the air flow distribution among the outlet pipes after achieving the desired coal flow rate distribution.